1. Field of the Invention
The present invention relates to apparatus and methods for determining physical characteristics of test materials. More particularly, the invention relates to apparatus and methods for determining physical characteristics relating to relatively thin test materials, for example, density and thickness characteristics thereof.
2. Description of the Prior Art
Nuclear radiation gauges for determining density and moisture characteristics of soil and asphaltic materials are well known. One example of such a gauge is described in U.S. Pat. No. 2,781,453. Such gauges employ the phenomenon of Compton scattering of gamma rays and are known by those skilled in the art as "scatter" gauges.
Gauges currently in use for measuring the density of soil, asphalt and other materials are most effective at measuring densities of materials over depths of approximately 4 to 6 inches. When the thickness of the test material is at least 4 to 6 inches, the prior gauges have been highly successful. However, increasing difficulty is encountered as the thickness of the test material decreases.
With the increasing cost of paving materials, the practice in maintaining and resufracing paved roadbeds has become one of applying overlays down to thicknesses on the order of one inch. With lifts of such a thickness range, prior density gauges are ineffective for measuring the density of the overlay applied. More particularly, such gauges are not capable of directly measuring the density of layers having a thickness of less than about four inches. The problem arises due to the depth of penetration of gamma rays. The gauge "sees" through the thin overlay so that the underlying pavement substantially influences the gauge reading.
Recognizing this limitation on the prior density gauges, efforts were made in the mid 1970's to establish a procedure for determining the density of thin overlays utilizing then existing gauges. A nomograph was developed that allowed approximation of the overlay density. However, in order to obtain the density of the overlay by the nomograph technique, it became necessary to know both the density of the underlying base and the thickness of the overlay. The technique was as follows. First, the operator determined the base density by taking nuclear density tests of the existing roadbed. Second, after the overlay pavement was applied and compacted, the overlay thickness was determined by taking a core sample, or like operation. Third, density tests were performed on top of the overlay to generate data. With the density measurement from the first test, the overlay thickness measurement and the density data from the second test, the density of the overlay could be approximated by reference to a nomograph.
A drawback of the prior art nomograph technique is that the underlying pavement is further compacted when the overlay is compacted, thereby introducing an inconsistency between the gauge reading of the underlying pavement density and its actual density after application of the overlay. Furthermore, it has proven difficult to take the second density reading (after the overlay is applied) at precisely the same location as the first reading. Lastly, the thickness of the overlay may vary between the sample location and the location where the nuclear gauge is placed for testing. In addition to the above problems, and perhaps most importantly, the nomograph technique required multiple steps, both before and after application of the overlay, as well as a destructive thickness measurement of the overlay.